System for diagnosing deterioration of catalyst

ABSTRACT

A diagnostic system in which the accuracy of the diagnosis of the deterioration state of a catalyst in a catalytic converter for cleaning the exhaust gas of an internal combustion engine is enhanced without incurring a rise in cost. The temperature of the catalyst is estimated using the operating-state signal (for example, the flow rate of intake air or the width of a fuel injection pulse) of the engine by a diagnostic device. The conversion efficiency of the catalyst calculated from the outputs of oxygen sensors is corrected using the estimated temperature by the diagnostic device. The deterioration state of the catalyst is diagnosed on the basis of the corrected temperature by the diagnostic device.

This is a continuation application of application Ser. No. 08/233,398,filed Apr. 26, 1994 U.S. Pat. No. 5,526,643.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diagnostic device for a catalyticconverter which is employed for treating exhaust gas emitted from aninternal combustion engine. More particularly, it relates to acatalyst-deterioration diagnostic system which can diagnose the state ofa catalyst by the use of the preset operating-state index of an internalcombustion engine and can report the diagnosed result through a display,an alarm sound or the like to the driver of a vehicle having the systeminstalled therein.

2. Description of the Related Art

It has been known that, in making the exhaust gas of an internalcombustion engine innoxious through oxidizing and deoxidizing reactions,effectively the reactions are promoted by employing a catalyst whichcontains a plurality of kinds of catalytic components. In this regard,when the catalyst is used for a long term, it deteriorates into a lowerconversion efficiency. Therefore, such a step has heretofore been takenthat the conversion efficiency of the catalyst is checked mainly at thetime of a routine inspection, and that a catalytic converter includingthe catalyst is replaced when the regulated value of the conversionefficiency is not met. Accordingly, the conversion efficiency is notespecially checked till the routine inspection, and a vehicle in whichthe catalytic converter is installed runs under the unsatisfactoryinnoxious condition of the exhaust gas in some cases.

Such circumstances unfavorably lead to environmental pollution. Attemptshave therefore been made to check the performance or activity of thecatalyst at all times by any method, and to actuate a warning device ora countermeasure device when the catalyst is abnormal. Examples will bestated below.

Japanese Patent Application Publication (KOKOKU) No. 2896/1982 disclosesa technique wherein oxygen-percentage detectors are respectively mountedbefore and behind a catalytic converter so as to diagnose the state ofthe catalytic converter on the basis of the detected values of both thedetectors. Concretely, the detectors capable of detecting the oxygenconcentrations of exhaust gas are respectively disposed upstream anddownstream of the catalytic converter. When the oxygen concentration ofthe exhaust gas detected by either of the detectors has abnormallyfallen outside a preset range until the difference thereof from theoutput of the other detector has exceeded a preset magnitude, a signalis generated to actuate an alarm device or a device for taking steps torectify the abnormality.

In addition, according to a technique entitled "Apparatus for DetectingDeterioration of Catalyst" as disclosed in Japanese Patent ApplicationLaid-open No. 249320/1991, the correlation coefficient of the outputsignals of oxygen sensors mounted before and behind a catalyticconverter is calculated so as to diagnose the state of a catalyst withthe calculated correlation coefficient.

Further, a method which adopts a sensor for diagnosing the deteriorationof a catalyst is disclosed in Japanese Patent Application Laid-open No.165744/1988. With this method, the catalyst for combustion is soconstructed that a catalytic component is carried on a support having alarge number of through holes. A basic material in the shape of a rod isinserted into the through hole, and is used as the sensor. The state ofthe catalyst is diagnosed by measuring the resistance of the rod acrossboth the ends thereof. This method can diagnose the state of thecatalyst irrespective of the operating conditions of an engine.

In general, however, the activity of a catalyst varies greatly independency on the temperature thereof as illustrated in FIG. 12 of theaccompanying drawings, so that the conversion efficiency etc. of thecatalyst varies in accordance with the temperature thereof. It isaccordingly necessary for the precise diagnosis of the deteriorationstate of the catalyst to stipulate the temperature of the catalyst onthe occasion of the diagnosis or to correct a calculated conversionefficiency in accordance with the temperature of the catalyst at thetime of the diagnosis. To this end, however, a temperature sensor formonitoring the temperature of the catalyst is required, resulting in theproblem of rise in cost.

SUMMARY OF THE INVENTION

The present invention has for its object to provide acatalyst-deterioration diagnostic system which can precisely diagnosethe state of a catalyst in correspondence with the temperature thereof,without incurring a rise in cost.

In the first aspect of performance of the present invention, there isprovided a temperature estimation apparatus, comprising state detectionmeans for detecting a value of a state variable which correlates with atemperature of a part to have its temperature detected; memory means forprestoring therein correspondence information which indicatecorresponding relations between detected results of the state detectionmeans and temperatures of the part for the temperature detection; andestimation means for estimating the temperature of the part for thetemperature detection by referring to the correspondence information onthe basis of the detected result of the state detection means.

In this case, it is preferable that the part for the temperaturedetection is a catalyst which cleans exhaust gas of an engine; and thatthe state variable includes at least one member which is selected fromthe group consisting of a quantity of intake air, a quantity of fuelinjection and r.p.m. (revolutions per minute) of the engine.

In the second aspect of performance of the present invention, there isprovided a method of estimating a temperature of a catalyst, comprisingthe steps of prestoring corresponding relations between temperatures ofthe catalyst which cleans exhaust gas of an engine and values of a statevariable which correlates with the temperatures of the catalyst; anddetecting an actual value of the state variable, and then comparing thedetected value with the stored corresponding relations, therebyestimating the temperature of the catalyst.

In the third aspect of performance of the present invention, there isprovided a catalyst-deterioration diagnostic System for diagnosing adeterioration state of a catalyst, comprising index means for obtaininga value of an index (for example, a conversion efficiency of thecatalyst) which is used for deciding the deterioration state of thecatalyst; catalyst state estimation means for estimating a state of thecatalyst at a time at which the index means has obtained the indexvalue, as to a physical quantity which affects a catalytic action of thecatalyst; correction means for correcting the index value obtained bythe index means, to a value in a standard state of the catalystpreviously set as to the physical quantity, by the use of the estimatedresult of the catalyst state estimation means; and decision meansendowed with a preset criterion value, and for deciding thedeterioration state of the catalyst by comparing the index valuecorrected by the correction means, with the criterion value.

In the fourth aspect of performance of the present invention, there isprovided a catalyst-deterioration diagnostic system for diagnosing adeterioration state of a catalyst, comprising index means for obtaininga value of an index (for example, a conversion efficiency of thecatalyst) which is used for deciding the deterioration state of thecatalyst; decision means endowed with a preset criterion value, and fordeciding the deterioration state of the catalyst by comparing the indexvalue obtained by the index means, with the criterion value; catalyststate estimation means for estimating a state of the catalyst at a timeat which the index means has obtained the index value, as to a physicalquantity which affects a catalytic action of the catalyst; andsuspension means endowed with a predetermined range concerning thephysical quantity, and for causing the decision means to suspend thedecision on condition that a value of the physical quantity obtained bythe catalyst state estimation means is outside the predetermined range.

In each of the third and fourth aspects of performance, thecatalyst-deterioration diagnostic system may well be so constructed thatthe catalyst serves to eliminate noxious substances which are containedin exhaust gas of an engine, and that the catalyst state estimationmeans includes operating-situation detection means for detecting a valueof a state variable of the engine as correlates with the physicalquantity, memory means for storing therein correspondence informationwhich indicate correlations between values of the state variable andthose of the physical quantity, and arithmetic means for determining avalue of the physical quantity by referring to the correspondenceinformation on the basis of the detected result of theoperating-situation detection means.

In the fifth aspect of performance of the present invention, acatalyst-deterioration diagnostic system for diagnosing a deteriorationstate of a catalyst, wherein the catalyst serves to eliminate noxiouscomponents which are contained in exhaust gas of an engine, comprises:index means for obtaining a value of an index which is used for decidingthe deterioration state of the catalyst; operating-situation detectionmeans for detecting a value of that state variable of the engine whichcorrelates with a physical quantity affecting a catalytic action of thecatalyst; correction means for correcting the index value obtained bythe index means, to a value in a standard state of the catalystpreviously set as to the physical quantity, by the use of the detectedresult of the operating-situation detection means; and decision meansendowed with a preset criterion value, and for deciding thedeterioration state of the catalyst by comparing the index valuecorrected by the correction means, with the criterion value.

In the sixth aspect of performance of the present invention, acatalyst-deterioration diagnostic system for diagnosing a deteriorationstate of a catalyst, wherein the catalyst serves to eliminate noxiouscomponents which are contained in exhaust gas of an engine, comprises:index means for obtaining a value of an index which is used for decidingthe deterioration state of the catalyst; decision means endowed with apreset criterion value, and for deciding the deterioration state of thecatalyst by comparing the index value obtained by the index means, withthe criterion value; operating-situation detection means for detecting avalue of that state variable of the engine which correlates with aphysical quantity affecting a catalytic action of the catalyst; andsuspension means endowed with a predetermined range concerning the statevariable, and for causing the decision means to suspend the decision oncondition that the value of the state variable detected by theoperating-situation detection means is outside the predetermined range.

In each of the fourth and sixth aspects of performance, it is preferablethat the catalyst-deterioration diagnostic system further comprisesalarm means for giving an alarm for the suspension of the decision whenthe decision has been suspended by the suspension means.

In the seventh aspect of performance of the present invention, acatalyst-deterioration diagnostic system for diagnosing a deteriorationstate of a catalyst, wherein the catalyst serves to eliminate noxiouscomponents which are contained in exhaust gas of an engine, comprises:index means for obtaining a value of an index which reflects thedeterioration state of the catalyst; operating-situation detection meansfor detecting a value of that state variable of the engine whichcorrelates with a physical quantity affecting a catalytic action of thecatalyst; correction means for correcting the value detected by theoperating-situation detection means, to a value in a standard state ofthe catalyst previously set as to the index, by the use of the indexvalue obtained by the index means; and decision means endowed with apreset criterion value, and for deciding the deterioration state of thecatalyst by comparing the value corrected by the correction means, withthe criterion value.

In each of the aspects of performance mentioned above, it is allowedthat the physical quantity is a temperature of the catalyst; and thatthe state variable includes at least one member which is selected fromthe group consisting of a quantity of intake air, a quantity of fuelinjection and r.p.m. (revolutions per minute) of the engine.

According to the present invention thus far summarized, the temperatureof the catalyst can be known without using a thermometer. Moreover,since the deterioration state of the catalyst is decided after thecorrection of the calculated conversion efficiency, it is diagnosedprecisely.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of the presentinvention;

FIG. 2 is a block diagram showing the internal construction of adiagnostic device;

FIG. 3 is a graph showing the general relationship between the flow rateof intake air and the temperature of a catalyst;

FIG. 4 is a graph showing the relationship between the temperature of acatalyst and the conversion efficiency thereof;

FIG. 5 is a graph showing the concept of a correction for the conversionefficiency;

FIG. 6 is a graph showing an example of a correction coefficient whichis used for the correction of the conversion efficiency;

FIG. 7 is a graph showing another example of the correction coefficient;

FIG. 8 is a graph showing still another example of the correctioncoefficient;

FIG. 9 is a graph showing the general relationship between the quantityof fuel injection and the temperature of the catalyst;

FIG. 10 is a graph showing the general relationship between therevolution speed of an engine and the temperature of the catalyst;

FIG. 11 is a graph showing an example in which the deterioration stateof the catalyst is decided on the basis of the value of the catalysttemperature; and

FIG. 12 is a graph showing the temperature characteristics of theactivity of the catalyst.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described in conjunctionwith the accompanying drawings.

First of all, an engine system to which a catalyst diagnostic system inthis embodiment is applied will be outlined with reference to FIG. 1. Itis to be understood, however, that the object of the application of thepresent invention is not restricted to the exemplified engine system.

Connected to the engine 2 of the engine system are an intake pipe 30 forintroducing air, and an exhaust pipe 32 for emitting a gas aftercombustion, as exhaust gas. A catalytic converter 4 is incorporatedmidway of the exhaust pipe 32. The catalytic converter 4 receivestherein a catalyst 40 which serves to eliminate noxious substances, suchas unburnt hydrocarbons (HC) and nitrogen oxides (NO_(x)), contained inthe exhaust gas. A ternary catalyst is often used as the catalyst 40,but the present invention is not restricted thereto. A temperaturesensor 16 is disposed on the exit side of the catalyst 40. In addition,the engine 2 is furnished with a fuel injector 12 which serves to injectfuel into the air introduced through the intake pipe 30, therebypreparing mixture.

Further, in order to control the engine system, the constituentsmentioned above are respectively furnished with sensors for detectingthe states thereof. By way of example, the intake pipe 30 is furnishedwith a flow-rate sensor 17 for detecting the quantity of intake air, andthe engine 2 with an r.p.m. (revolutions per minute) sensor 18 fordetecting the revolution speed or number thereof.

The air introduced through the intake pipe 30 and the fuel injected bythe fuel injector 12 are turned into the mixture, which is burnt in thecylinder of the engine 2. Thereafter, the gas after the combustion isemitted as the exhaust gas through the exhaust pipe 32. The unburnthydrocarbons (HC), nitrogen oxides (NO_(x)), etc. contained in theexhaust gas are treated by the catalyst 40 received in the catalyticconverter 4.

The catalyst diagnostic system in this embodiment will be described.

As already stated in the section of the related art, the conversionefficiency of a catalyst varies depending upon the temperature thereof,so that it needs to be calculated subject to the previous stipulation ofthe temperature state of the catalyst. Meanwhile, the temperature of thecatalyst 40 during the operation of the engine 2 varies basically inaccordance with the quantity of heat which the catalyst 40 receives fromthe exhaust gas. This embodiment features that, with note taken of theabove aspect of the temperature variation of the catalyst 40, the outputsignals of the several sensors (the flow-rate sensor 17, the r.p.m.sensor 18, etc.) and the width T_(P) of the fuel injection pulse of thefuel injector 12 as indicative of the operating situation of the engine2 are monitored as state variables, whereupon a predeterminedcalculation is executed with at least one of the state variables,thereby estimating the temperature state of the catalyst 40 thoughapproximately. Also, this embodiment features that the diagnosis of thedeterioration of the catalyst is corrected or suspended in accordancewith the estimated result.

By the way, in this specification, the expression "conversionefficiency" shall be defined by Equation (1) given below. Besides, anexpression "degree of deterioration" shall be defined by Equation (2)given below. ##EQU1##

Now, the construction and operation of this embodiment will be describedconcretely.

The catalyst diagnostic system is mainly constructed of a diagnosticdevice 8, sensors for detecting the operating situation of the engine 2,and oxygen sensors 6a, 6b (refer to FIG. 1).

The oxygen sensors 6a and 6b function to detect the oxygenconcentrations of the exhaust gas, and are made of zirconia (ZrO₂) inthis embodiment. It is to be understood, however, that the material ofthe oxygen sensors 6a and 6b are not restricted to the adopted one. Theoxygen sensor 6a is mounted in correspondence with the entrance part ofthe catalytic converter 4 in order to detect the oxygen concentration ofthe exhaust gas before being treated by the catalyst 40. On the otherhand, the oxygen sensor 6b is mounted in correspondence with the exitpart of the catalytic converter 4 in order to detect the oxygenconcentration of the exhaust gas after having been treated by thecatalyst 40.

The sensors for detecting the operating situation of the engine 2include a sensor for detecting the quantity of intake air, a sensor fordetecting the r.p.m. (revolutions per minute) of the engine 2, and soforth. Since, however, the flow-rate sensor 17, r.p.m. sensor 18, etc.stated before can be utilized as the sensors of the diagnostic systemwithout any change, these sensors for the diagnostic use need not bedisposed anew (naturally, they need to be disposed anew in a case wherethe engine 2, etc. are not furnished with the sensors 17, 18, etc.).

As illustrated in FIG. 2, the diagnostic device 8 includes thereincatalyst state estimation means 80, conversion efficiency calculationmeans 82 and decision means 84. The catalyst state estimation means 80is furnished with a temperature map 800.

The catalyst state estimation means 80 executes a predeterminedcalculation with the output signal 170 (indicative of the quantity Qa ofintake air) of the flow-rate sensor 17, thereby estimating thetemperature state of the catalyst 40. In this embodiment, the estimationis based on the output signal 170 of the flow-rate sensor 17. As analternative contrivance, however, the temperature state of the catalyst40 can be estimated even with any of the width T_(P) of the fuelinjection pulse of the fuel injector 12, namely, the output signal 120of the fuel injector 12 as shown in FIG. 1 (in general, a fuel injectoradjusts the quantity of fuel injection by adjusting the width of a fuelinjection pulse), the output signal 180 (indicative of the revolutionspeed Ne of the engine 2) of the r.p.m. sensor 18, the velocity of anautomobile in which the engine system is installed, and so forth.Incidentally, an expedient for estimating the catalyst temperature willbe explained in detail later.

The conversion efficiency calculation means 82 calculates the conversionefficiency at which the catalyst 40 converts the noxious substances intoinnoxious substances. The calculation of the conversion efficiency iseffected using the output signals 60a, 60b of the respective oxygensensors 6a, 6b. Since a method for the calculation is not, in itself,essential to this embodiment and is a technique having already beenknown, it shall not be especially explained here. Applicable as thecalculation method is, for example, one disclosed in Japanese PatentApplication No. 338220/1991. However, the calculation method forcomputing the conversion efficiency in the present invention is notrestricted to this example, but the conversion efficiency may well becalculated by any other method.

In addition to the function of calculating the conversion efficiency bythe use of the actual measurement data of the oxygen concentrations, theconversion efficiency calculation means 82 has the function ofcorrecting the calculated value of the conversion efficiency inaccordance with the temperature of the catalyst 40 on each occasion.This conversion efficiency calculation means 82 is endowed withtemperatures (T_(X1) and T_(X2) to be mentioned later) which serve ascriteria concerning whether or not the calculated value of theconversion efficiency is to be corrected and whether or not the state ofthe catalyst is to be diagnosed. A method for the correction will beexplained in detail later.

The decision means 84 is previously endowed with a standard limit valueC_(ST) indicating that limit of the deterioration at which the catalyst40 (catalytic converter 4) must be replaced. Thus, it compares thecalculated result (=the conversion efficiency or the corrected valuethereof) of the conversion efficiency calculation means 82 with thestandard limit value C_(ST), thereby deciding the deterioration state ofthe catalyst 40. In a case where the necessity of the replacement of thecatalyst 40 has been diagnosed as the result of the decision, thedecision means 84 functions to actuate alarm means 14 and to givewarning to the driver of the automobile as will be explained later.Besides, in a case where the conversion efficiency calculation means 82produces a result indicative of the impossibility of the diagnosis, forexample, where the temperature estimated by the catalyst stateestimation means 80 is lower than the preset one, the decision means 84does not render the decision.

By the way, the above expression "that limit of the deterioration atwhich the catalyst 40 must be replaced" does not signify that lowerlimit of the conversion efficiency which is indiscriminately set, but itsignifies "that limit of the deterioration at which the exhaust gas canbe cleaned up to, at least, a state capable of satisfying the p.p.m.(parts per million) regulations thereof stipulated legally". That is,the limit of the deterioration is set for each individual engine systemin relation to the capacity of the catalytic converter 4. In the case ofthe engine system which is furnished with the catalytic converter 4 ofvery large capacity beforehand, accordingly, the catalyst 40 is notalways decided as requiring replacement even when the conversionefficiency has lowered drastically (for example, even when theconversion efficiency has become 10 [%]). To the contrary, in the caseof the engine system which employs the catalytic converter 4 havingalmost no margin in capacity, the catalyst 40 can be decided asrequiring replacement even when the conversion efficiency has loweredonly by a slight amount (for example, even when the conversionefficiency has become 90 [%]). However, the decision of the catalyststate is not restricted to the above method, but it may well be renderedin relation to the value of the conversion efficiency setindiscriminately. Needless to say, however, the p.p.m. regulations ofthe exhaust gas need to be met even in this case. Such a requisite comesfrom the viewpoints of observing laws and preserving the environment,and does not come from the technical viewpoint of the present invention.

Since the conversion efficiency of the catalyst 40 varies depending uponthe temperature thereof, naturally the comparison of the conversionefficiency with the standard limit value C_(ST) needs to be made at acertain temperature specified as a criterion (hereinbelow, termed the"standard decision temperature T_(S) ") on which the setting of thestandard limit value C_(ST) is premised. Accordingly, a correctioncoefficient K_(XX) to be stated later is set assuming that theconversion efficiency after the correction becomes a value at thestandard decision temperature T_(S). The decision means 84 may wellpossess the standard decision temperature T_(S) along with the standardlimit value C_(ST).

The diagnostic device 8 in practical use is configured of an arithmeticcircuit such as a microcomputer, and a memory in which programs are heldstored. Various information items necessary for the diagnosis, whichcontain the temperature map 800 and values C_(ST), T_(S), T_(X1) andT_(X2) stated above and correction coefficients to be stated later, arestored in the memory of the diagnostic device 8 beforehand.

The alarm means 14 reports the necessity of the replacement of thecatalyst 40 to the driver of the automobile by, e.g., sounding an alarmor lighting up a warning lamp in accordance with a signal 140 which isdelivered from the diagnostic device 8 when the catalyst replacement isrequired.

An expression "standard state" corresponds to the state of the standarddecision temperature T_(S) in this embodiment, and an expression"criterion value" to the standard limit value C_(ST). An expression"predetermined range concerning the physical quantity" corresponds to atemperature range which is not lower than the temperature T_(X1) (orT_(X2)). "Index means", "correction means" and "suspension means" areimplemented by the conversion efficiency calculation means 82 in thisembodiment. "Operating-situation detection means" includes the flow-ratesensor 17, etc. An expression "correspondence information" signifies thetemperature map 800. "Arithmetic means" and "memory means" correspond tothe microcomputer and the memory which constitute the diagnostic device8, respectively.

Now, the operation of this embodiment will be described. First of all,the calculation of the catalyst state estimation means 80 for theestimation of the temperature state of the catalyst 40 will be explainedin detail.

The relationship between the flow rate of intake air and the temperatureof a catalyst is generally such that, as illustrated in FIG. 3, thecatalyst temperature rises with increase in the flow rate of intake air.This is based on the fact that the temperature of the catalyst variesdepending upon the temperature, flow rate etc. of exhaust gas (that is,the quantity of combustion or the quantity of generated heat within anengine), and that the quantity of combustion, etc., in turn, correspondto the flow rate of intake air.

The catalyst state estimation means 80 is endowed beforehand with thetemperature map 800 which indicates the relationship between the averagevalue of air flow rates within a predetermined time period and thetemperature of the catalyst 40.

Using the output values 170 of the flow-rate sensor 17, the catalyststate estimation means 80 calculates the average value of air flow rateswithin a predetermined time period in the past (needless to say, thispredetermined time period is equal in length to the time perioddetermined for the calculation of the average value of air flow rates inthe preparation of the temperature map 800). Subsequently, theestimation means 80 refers to the temperature map 800 and estimates atemperature held in correspondence with the calculated average value, asthe temperature of the catalyst 40 at that time.

Herein, the length of the time period for calculating the average valueis determined for each individual engine system in relation to thethermal capacity and specific heat of the catalyst 40, the quantity ofthe exhaust gas emitted from the engine, and so forth. The reasontherefor is that the temperature of the catalyst 40 is determined by thequantity of heat accumulated in the catalyst 40, and that the quantityof the accumulated heat is, in turn, determined by the magnitudes of thequantities of heat flowing into and out of the catalyst 40, thedifference and ratio between the outflow and inflow, and so forth.

Next, there will be explained that correction of the conversionefficiency which the conversion efficiency calculation means 82 executesusing the estimated temperature of the catalyst state estimation means80. As illustrated in FIG. 4, the aspect of the variation of theactivity of the catalyst 40 differs greatly, depending upon thetemperature regions of this catalyst. In the region above thetemperature T_(X2), the catalyst 40 demonstrates its activitysufficiently, and the magnitude of the activity is substantiallyconstant irrespective of temperature values. In contrast, in the regionbelow the temperature T_(X1), the catalyst 40 is hardly active at all.Besides, in the region between the temperatures T_(X1) and T_(X2), thecatalyst activity varies greatly in correspondence with temperaturevalues.

Accordingly, the conversion efficiency calculation means 82 executesdifferent processes in accordance with which of the three regions thetemperature of the catalyst 40 lies in. Although the concrete values ofthe temperatures T_(X1) and T_(X2) differ depending upon the kind of thecatalyst 40, they are about 350 [°C.] and about 450 [°C.], respectively,in many kinds of catalysts. Incidentally, the values of the temperaturesT_(X1) and T_(X2) are bestowed on the conversion efficiency calculationmeans 82 beforehand.

In the region III whose temperature is higher than the value T_(X2), thecalculation means 82 delivers the conversion efficiency obtained withactual measurement data on each occasion, to the decision means 84without any change. Since, as stated before, the method of calculatingthe conversion efficiency does not form the feature of the presentinvention, it shall not be explained in detail here.

In the region I whose temperature is lower than the value T_(X1), thecatalyst activity is hardly noticeable, and it is inappropriate todiagnose the conversion efficiency and the degree of deterioration bythe use of data measured under such a state. In this case, therefore,the calculation means 82 deems the diagnosis (or the correction) to beimpossible and supplies the decision means 84 with an output indicatingto that effect.

When the temperature of the catalyst 40 lies in the region II whichextends between the temperatures T_(X1) and T_(X2), the conversionefficiency calculation means 82 corrects the conversion efficiencycalculated on the basis of actual measurement data, to a value at thestandard decision temperature T_(S) on which the setting of the standardlimit value C_(ST) (possessed by the decision means 84) is premised.Thereafter, it delivers the corrected value to the decision means 84.

The correction is done by multiplying the calculated conversionefficiency by the correction coefficient. By way of example, let'sconsider a case illustrated in FIG. 5 where the temperature estimated bythe catalyst state estimation means 80 is T_(XX) and where theconversion efficiency calculated on the basis of the actual measurementdata by the conversion efficiency calculation means 82 is C_(XX) (inother words, a case which corresponds to the position of a point P shownin FIG. 5). In this case, a value C_(XS) obtained by multiplying theconversion efficiency C_(XX) by the correction coefficient K_(XX) at thetemperature T_(XX) as indicated by Equation (3) below is delivered tothe decision means 84 as the conversion efficiency at the standarddecision temperature T_(S) (corresponding to a point P'). Incidentally,although a curve m indicating the variation of the conversion efficiencyof the catalyst 40 is depicted in FIG. 5, the depiction is merelyintended to facilitate understanding, and the diagnostic device 8 doesnot have the data of the whole curve m.

    C.sub.XS =C.sub.XX ·K.sub.XX                      (3)

The correction coefficient K_(XX) is set on the basis of empirical data,assuming that the conversion efficiency C_(XS) after the correctionbecomes the value at the standard decision temperature T_(S). Thecorrection coefficient K_(XX) is bestowed on the conversion efficiencycalculation means 82 beforehand (refer to FIG. 6). Originally, precisecorrections cannot be made unless the gradient etc. of a segment nindicative of the correction coefficient K_(XX) is altered in accordancewith the deterioration state of the catalyst 40. However, few problemsarise in practical use when the correction coefficient is set on thepremise of the deterioration of the catalyst intermediate between thenormal state of the catalyst having undergone no deterioration and thedeterioration state of the catalyst requiring the replacement thereof.Although the correction coefficient K_(XX) is rectilinearly depicted inFIG. 6, it is needless to say that the present invention is notrestricted to such a rectilinear correction coefficient. As statedabove, the optimum correction coefficient is set on the basis of theempirical data.

In this embodiment, the standard decision temperature T_(S) is equalizedto the temperature T_(X2) in order that the decision means 84 may bepermitted to render the decisions by the use of the identical standardlimit value C_(ST) in the regions III and II.

In the region III, the conversion efficiency calculated on the basis ofonly the actual measurement data is directly output without beingcorrected. This aspect can also be considered as the state in which thecorrection coefficient is 1 (unity). Accordingly, when the correctioncoefficient is defined as illustrated in FIG. 7, the regions II and IIIcan be handled without any discrimination. Further, in the case wherethe conversion efficiency is corrected also in the region III in thismanner, only one standard limit value C_(ST) may well be set withoutbringing the standard decision temperature T_(S) and the temperatureT_(X2) into agreement. The reason therefor is that, with some ways ofsetting the correction coefficient, the conversion efficiency calculatedin the region III can also be corrected down to the standard decisiontemperature T_(S). In this case, the correction coefficient becomessmaller than 1 (unity) in the region III (refer to FIG. 8).

The method of the correction is not restricted to the above one, but thecorrection of the conversion efficiency may well be executed using, forexample, an approximate formula which is empirically obtained.

Further, the execution of the correction of the conversion efficiencymay well be substituted by a contrivance in which the relationshipbetween the temperature in the region II and the conversion efficiency(in the state in which the catalyst 40 has undergone no deterioration)is held in the form of a map, and in which the conversion efficiency atthe temperature estimated by the catalyst state estimation means 80 isread by referring to the map. In this case, the decision means 84compares the conversion efficiency at the estimated temperature readfrom the map, with the conversion efficiency at the estimatedtemperature calculated on the basis of the actual measurement data bythe conversion efficiency calculation means 82. This contrivance bringsforth the advantages that a higher processing speed can be attained, andthat a more precise decision can be rendered. The map is bestowed on thedecision means 84. The conversion efficiency calculation means 82 needsonly to calculate the conversion efficiency at the temperature estimatedby the catalyst state estimation means 80 and to deliver the calculatedvalue to the decision means 84. The map may well be bestowed on theconversion efficiency calculation means 82. In this case, thecalculation means 82 supplies the decision means 84 with both the valueread from the map and the value calculated from the actual measurementdata.

Alternatively, the correction of the conversion efficiency thus farexplained can be dispensed with in such a way that the decision by thedecision means 84 is rendered in only the region III and is suspended inthe regions I and II. As in the process concerning the region I, theconversion efficiency calculation means 82 determines whether or not thedecision is to be suspended, and it supplies the decision means 84 withan output indicating to that effect.

In this embodiment, the temperature of the catalyst 40 is estimatedusing the value of the flow rate of intake air (Qa indicated by theoutput signal 170 of the flow-rate sensor 17). As stated before,however, the catalyst temperature can be similarly estimated with theaverage value of the quantity of fuel injection within a predeterminedtime period in the past (this average value can be readily known from,for example, a computation utilizing the width T_(P) of a fuel injectionpulse as indicated by the output signal 120 of the fuel injector 12) orthe average value of the revolution speed (r.p.m.) Ne of the engine 2within a predetermined time period in the past (this average value canbe readily known from the output signal 180 of the r.p.m. sensor 18).Further, the estimation of the catalyst temperature is permitted bycomprehensively considering the velocity of the automobile and the stageof a reduction gear.

As illustrated in FIG. 9 by way of example, the relationship between thequantity of fuel injection and the catalyst temperature is such that thetemperature of the catalyst 40 becomes higher as the quantity of fuelinjection within a predetermined time period is larger. As similarlyillustrated in FIG. 10, the relationship between the revolution speed(r.p.m.) of the engine 2 and the catalyst temperature is such that thetemperature of the catalyst 40 becomes higher as the revolution speed ishigher. Such tendencies are observed for the reason that, likewise tothe flow rate of intake air, the quantity of fuel injection and therevolution speed have somewhat corresponding relations with the quantityof combustion proceeding in the combustion chamber of the engine 2.Needless to say, FIGS. 9 and 10 correspond to FIG. 3 in the foregoingembodiment, and similar techniques are applicable concerning theestimation of the catalyst temperature, the correction of the conversionefficiency, and so forth.

It has been explained above that the temperature is estimated using theaverage value within the predetermined time period in the past. Inprinciple, however, it is the same as the above contrivance to evaluatethe total quantity of intake air (the total quantity of fuel injection,or the total number of revolutions) within the predetermined time periodin the past, and to estimate the temperature of the catalyst 40 by theuse of the value of the total quantity of intake air (or the like) as itis. In this case, an arithmetic operation for calculating the averagevalue is dispensed with, so that a higher processing speed is attained.

Besides, in a case where the operating situation of the engine 2 is verysteady, or in the engine system wherein the catalyst temperature reachesthe region III mostly in the state in which the engine 2 is inoperation, neither of the process of the calculation of the averagevalue and the measurement of the time period as in the foregoingcontrivances is required. Herein, it suffices to merely integrate thequantity of intake air and then diagnose the state of the catalyst 40when the integral value has reached a prescribed value.

In the embodiment described above, whether or not the replacement of thecatalyst 40 is necessary is decided by judging the magnitude of theconversion efficiency. To the contrary, however, the necessity of thereplacement of the catalyst 40 can also be decided by checking the valueof the catalyst temperature in the state in which the conversionefficiency assumes a standard decision conversion efficiency C_(S)specified as a criterion (refer to FIG. 11). In a case where thecatalyst temperature estimated by the catalyst state estimation means 80is higher than a standard limit temperature T_(ST) in the state in whichthe value of the conversion efficiency is equal to the standard decisionconversion efficiency C_(S), the decision means 84 decides that thereplacement of the catalyst 40 is required. Incidentally, the standarddecision conversion efficiency C_(S) serves equivalently to the standarddecision temperature T_(S) in the foregoing embodiment. Besides, thestandard limit temperature T_(ST) serves equivalently to the standardlimit value C_(ST) Of the conversion efficiency in the foregoingembodiment.

In a case where the value C_(YY) of the conversion efficiency calculatedon the basis of the actual measurement data by the conversion efficiencycalculation means 82 is unequal to the standard decision conversionefficiency C_(S), a corresponding temperature T_(YY) is corrected usingthe same technique as in the foregoing embodiment. Then, the decisionmeans 84 renders the decision in such a way that a temperature T_(YS)obtained by the correction of the temperature T_(YY) is compared withthe standard limit temperature T_(ST). An expression "standard statepreviously set as to the index" corresponds to the standard decisionconversion efficiency C_(S) stated here. An expression "criterion value"corresponds to the standard limit temperature T_(ST) stated here.

As thus far described, according to the embodiment, the temperature ofthe catalyst can be known without additionally providing a thermometer.Moreover, the deterioration state of the catalyst is decided aftercorrecting the calculated value of the conversion efficiency inaccordance with the temperature of the catalyst, so that the state ofthe catalyst can be precisely diagnosed.

Incidentally, the technique of the temperature estimation by thecatalyst state estimation means is extensively applicable, not only forestimating the temperature of the catalyst, but also for estimating thetemperatures of the engine itself and various parts of the exhaustsystem of the engine. Besides, the accuracy of the temperatureestimation is heightened when the quantity of intake air, the quantityof fuel injection, etc. are comprehensively included in the temperatureestimation.

The present invention is not restricted to the above case of utilizingthe conversion efficiency, but it is also applicable to the case ofmaking a diagnosis by the use of any parameter or the like whichreflects the deterioration state of the catalyst. Needless to say,however, the correcting method explained before needs to be modified oraltered in correspondence with the parameter or the like.

In the foregoing embodiment, the temperature of the catalyst 40 isestimated on the basis of the operating situation of the engine 2detected by the flow-rate sensor 17 or the like, and the judgement onthe propriety of the execution of the decision, the correction of theconversion efficiency, etc. are made on the basis of the estimatedtemperature. In an actual equipment, however, the judgement on thepropriety of the execution of the decision, the correction of theconversion efficiency, etc. may well be made in such a way that theoperating situation of the engine 2 detected by any of the sensors isused directly without the intervention of the parameter "temperature" asin the embodiment. By way of example, the decision may well be suspendedon condition that the average value of the quantity of intake air withina predetermined time period in the past is less than a certain value.

In the above, the functions of the catalyst state estimation means 80,conversion efficiency calculation means 82 and decision means 84 havebeen explained as being definitely separate. In actual fact, however,these means operate in close association, and the correspondingrelations of the aspects of performance with the appended claims are notstrict. Insofar as the functions are fulfilled of the entire diagnosticdevice 8, they may be allotted anyhow. By way of example, the functionas the "suspension means" may well be owned by the decision means 84.

What is claimed is:
 1. A catalyst-deterioration diagnostic system fordiagnosing a deterioration state of a catalyst, comprising:index meansfor obtaining a value of an index which is used for deciding thedeterioration state of the catalyst; calculation means for calculating avalue which correlates to a physical quantity affecting a catalyticaction of the catalyst, by use of a value of that state variable of anengine as an input; correct means for correcting said index value inaccordance with the calculated value obtained by said calculation means;and decision means endowed with a present criterion value, and fordeciding the deterioration state of the catalyst by comparing said indexvalue corrected by said correct means with said criterion value.
 2. Acatalyst-deterioration diagnostic system for diagnosing a deteriorationstate of a catalyst, comprising:index means for obtaining a value of anindex which is used for deciding the deterioration state of thecatalyst; calculation means for calculating a value which correlates aphysical quantity affecting a catalytic action of the catalyst, by useof a value of that state variable of an engine as an input; correctmeans for correcting a preset criterion value used for deciding thedeterioration of the catalyst in accordance with the calculated valueobtained by said calculation means; and decision means for deciding thedeterioration state of the catalyst by comparing said criterion valuecorrected by said correct means with said index value obtained by saidindex means.
 3. A catalyst-deterioration diagnostic system fordiagnosing a deterioration state of a catalyst, comprising:index meansfor obtaining a value of an index which is used for deciding thedeterioration state of the catalyst; catalyst state calculation meansfor calculating a state of the catalyst at a time at which said indexmeans has obtained the index value, as to a physical quantity whichaffects a catalytic action of the catalyst; correction means forcorrecting said index value obtained by said index means, to a value ina standard state of the catalyst previously set as to the physicalquantity, by the use of the estimated result of said catalyst statecalculation means; and decision means endowed with a preset criterionvalue, and for deciding the deterioration state of the catalyst bycomparing said index value corrected by said correction means, with saidcriterion value.
 4. A catalyst-deterioration diagnostic system fordiagnosing a deterioration state of a catalyst, comprising:index meansfor obtaining a value of an index which is used for deciding thedeterioration state of the catalyst; calculation means for calculating avalue which correlates to a physical quantity affecting a catalyticaction of the catalyst, by use of a value of that state variable of anengine as an input; correct means for correcting said index value inaccordance with a characteristic which is substantially opposed to acatalyst activity corresponding to a physical quantity affecting acatalytic action of the catalyst; and decision means endowed with apreset criterion value, and for deciding the deterioration state of thecatalyst by comparing said index value corrected by said correct meanswith said criterion value.
 5. A catalyst-deterioration diagnostic systemfor diagnosing a deterioration state of a catalyst, comprising:indexmeans for obtaining a value of an index which is used for deciding thedeterioration state of the catalyst; calculation means for calculating avalue which correlates to a physical quantity affecting a catalyticaction of the catalyst, by use of a value of that state variable of anengine as an input; correct means for correcting a preset criterionvalue used for deciding the deterioration of the catalyst in accordancewith a characteristic which is substantially opposed to a catalystactivity corresponding to a physical quantity affecting a catalyticaction of the catalyst; and decision means for deciding thedeterioration state of the catalyst by comparing said index valueobtained by said index means with said criterion value corrected by saidcorrect means.
 6. A catalyst-deterioration diagnostic system fordiagnosing a deterioration state of catalyst, comprising:index means forobtaining a value of an index which is used for deciding thedeterioration state of the catalyst; calculation means for calculating avalue which correlates a physical quantity affecting a catalytic actionof the catalyst, by use of a value of that state variable of an engineas an input; decision means for deciding the deterioration state of thecatalyst by comparing said index value with a preset criterion value;and suspension means for causing said decision means to suspend thedecision on condition that the calculated value which correlates to thephysical quantity affecting the catalytic action of the catalyst doesnot fall within a predetermined range.
 7. A catalyst-deteriorationdiagnostic system for diagnosing a deterioration state of catalyst,comprising:index means for obtaining a value of an index which is usedfor deciding the deterioration state of the catalyst; calculation meansfor calculating a value which correlates a physical quantity affecting acatalytic action of the catalyst, by use of a value of that statevariable of an engine as an input; decision means for deciding thedeterioration state of the catalyst by comparing said index value with apreset criterion value; and suspension means for causing said decisionmeans to temporarily interrupt the decision on condition that thecalculated value which correlates to the physical quantity affecting thecatalytic action of the catalyst does fall within a predetermined range.8. A catalyst-deterioration diagnostic system for diagnosing adeterioration state of catalyst, comprising:index means for obtaining avalue of an index which is used for deciding the deterioration state ofthe catalyst; calculation means for calculating a value which correlatesa physical quantity affecting a catalytic action of the catalyst, by useof a value of that state variable of an engine as an input; decisionmeans for deciding the deterioration state of the catalyst by comparingsaid index value with a preset criterion value; and cancel means forcanceling the decision obtained by said decision means on condition thatthe calculated value which correlates to the physical quantity affectingthe catalytic action of the catalyst does not fall within apredetermined range.
 9. A catalyst-deterioration diagnostic method fordiagnosing a deterioration state of a catalyst, comprising the stepsof:obtaining a value of an index which is used for deciding thedeterioration state of the catalyst; calculating a value whichcorrelates to a physical quantity affecting a catalytic action of thecatalyst, by use of a value of that state variable of an engine as aninput; correcting said index value in accordance with the calculatedvalue obtained by the calculating steps; and deciding the deteriorationstate of the catalyst by comparing the corrected index value a presetcriterion value.
 10. A catalyst-deterioration diagnostic method fordiagnosing a deterioration state of a catalyst, comprising stepsof:obtaining a value of an index which is used for deciding thedeterioration state of the catalyst; calculating a value whichcorrelates to a physical quantity affecting a catalytic action of thecatalyst, by use of a value of that state variable of an engine as aninput; correcting a preset criterion value used for deciding thedeterioration of the catalyst in accordance with the calculated valueobtained by the calculating step; and deciding the deterioration stateof the catalyst by comparing the corrected criterion value with theindex value.
 11. A catalyst-deterioration diagnostic method fordiagnosing a deterioration state of a catalyst, comprising the stepsof:obtaining a value of an index which is used for deciding thedeterioration state of the catalyst; calculating a value whichcorrelates to a physical quantity affecting a catalytic action of thecatalyst, by use of a value of that state variable of an engine as aninput; correcting said index value in accordance with a characteristicwhich is substantially opposed to a catalyst activity corresponding to aphysical quantity affecting a catalytic action of the catalyst; anddeciding the deterioration state of the catalyst by comparing thecorrected index value corrected with a preset criterion value.
 12. Acatalyst-deterioration diagnostic method for diagnosing a deteriorationstate of a catalyst, comprising steps of:obtaining a value of an indexwhich is used for deciding the deterioration state of the catalyst;calculating a value which correlates to a physical quantity affecting acatalytic action of the catalyst, by use of a value of that statevariable of an engine as an input; correcting a preset criterion valueused for deciding the deterioration of the catalyst in accordance with acharacteristic which is substantially opposed to a catalyst activitycorresponding to a physical quantity affecting a catalytic action of thecatalyst; and deciding the deterioration state of the catalyst bycomparing the index value with the corrected criterion value.